1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, a printing machine, a fax machine or a multi-function peripheral providing multiple functions of those machines, and also relates to a fixing device provided in such an image forming apparatus.
2. Description of the Related Art
Conventionally, as to a fixing device including in an image forming apparatus (e.g. a copying machine or a printing machine), a technology is known that provides a separating member (e.g. a separating plate) to oppose a fixing member in order to prevent a recording medium after a fixing operation from winding around the fixing member (see Patent Document 1, for example).
In a fixing device, a fixing member (e.g. a fixing roller or a fixing belt) abuts a pressure member (e.g. a pressure roller, a pressure belt or a pressure pad) by a pressure means, and a nip portion (fixing nip portion) is formed between these two members. The fixing member is heated by a heating means such as a heater or an exciting coil, and a toner image having been transferred onto a recording medium at a transfer unit is fed into the nip portion and then fixed onto the recording medium by heat and pressure.
In such a fixing device, a separating member is provided on the downstream side in the moving direction of the fixing member in relation to the nip portion in a manner to oppose the fixing member across a small gap. Even if a recording medium immediately after the fixing operation adheres to the fixing member, the separating member forces the recording medium to separate from the fixing member. Thus, the recording medium does not end up winding around the fixing member, and is guided to its conveyance path.
In Patent Document 1, for example, projections (position setting members) formed by a rolling process or a bending process are provided at both ends of the separating plate (separating member) in the width direction. The separating plate is urged toward the fixing member by a spring so as to bring the projections to abut the fixing member. In this way, a small gap is formed between the separating plate and the fixing member.
On the other hand, Patent Document 2, for example, discloses a technology that grounds, via a resistance, a driving roller of a transfer carrying belt and a front guide situated before the fixing member in order to prevent drops in a transfer bias voltage and also prevent an unfixed toner image on a recording medium from being dispersed by static electricity.
Patent Document 3, for example, discloses a technology that applies a bias between the fixing member and the pressure member in order to prevent current flowing them.
Patent Document 1: Japanese Laid-open Patent Application Publication No. 2006-171551
Patent Document 2: Japanese Laid-open Patent Application Publication No. 2003-107919
Patent Document 3: Japanese Laid-open Patent Application Publication No. 2003-316187
As to conventional fixing devices described above, in the case where the length of the recording medium along the conveyance direction is long in relation to the distance between the separating member and the transfer unit performing the toner-image transfer operation (i.e. in the case where the separating operation by the separating member is being carried out at the front end of the recording medium while the toner-image transfer operation by the transfer unit is being carried out at the rear end of the recording medium), a transfer current applied to the transfer unit flows to the separating member via the recording medium, thereby sometimes resulting in transfer defects at the transfer unit, such as defects in image transfer density. This problem becomes eminent particularly when the moisture content of the recording medium is high (for example, in the case of using a recording medium having a moisture content of 10% or more after having been stored for a long period of time in a high-humidity environment) since such a recording medium is prone to passing an electric current.
In particular, the separating member may come in direct contact with a transfer surface (i.e. fixing surface) of the recording medium with a large force when performing its function, and therefore the conventional fixing devices are subject to the above problem. In order to solve this problem, the separating member may be grounded via an electric resistor. However, providing an electric resistor leads to an increase in cost and size of the device. Particularly, in the case where the separating member is rotatably supported against the frame of the fixing device, as in Patent Document 1, connecting the electric resistor may interrupt the rotation of the separating member and result in changing the gap between the separating member and the fixing member. Or, enough space for providing the electric resistor may not be reserved.
Note that the above problem is not limited to the separating member of a fixing device. The same problem arises with, under the above-mentioned conditions, an opposing member that is disposed in the conveyance path of a recording medium so as to face the recording medium.